DE4419454A1 - Magnet system for NMR tomography - Google Patents

Abstract

The magnet system has two magnetic coils (S1,S2) on either side of the investigation vol. and coaxial wrt. a z-axis passing through the centre of the vol. The coils carry a d.c. current in the same direction during operation. Two ferromagnetic rings (R11,R12;R21,R22) are arranged coaxially wrt. each magnetic coil and closer to the centre of the vol. than the adjacent coil. The dimensions and positions of the rings are such that the magnetic field in the centre of the investigation vol. is homogenised at least approximately up to the 10th or pref. 12th order with current flowing in the magnetic coils.

Description

This application is an additional application for P 44 12 755.3 dated April 13. 1994, to which reference is hereby made in full.

The invention relates to a magnetic system for nuclear magnetic resonance (NMR) tomography for generating an approximately homogeneous at least 8th order static magnetic field within an examination volume with two coaxial with respect to a running through the center of the examination volume z-axis, both sides of the examination volume Magnetic coils (S₁, S₂), which assume a z position with respect to the center of the examination volume z _S1 or z _S2 , have an average distance r _S1 or r _S2 from the z axis and are flowed through in the same direction during operation by direct current.

Such a magnet system is known for example from DE 39 07 927 A1.

NMR tomography systems find their main medical application in the tomogra for non-invasive examination of patients. The interpretation of the Systems are ensured that both the field strength and the homogeneity the static magnetic field is as high as possible, so u. a. a high resolution he produced sectional images is achieved. The weight of the magnet system should be as possible be ring to avoid location problems. To verify claustrophobia of the patient avoid the overall length as little as possible, or there should be side openings, through the u. U. even surveillance and therapy measures during the image may be possible. Of course, it is also desirable, if possible use less expensive materials and use as little energy as possible during operation need.  

In DE 36 16 078 C2 an electromagnetic system for magnetic resonance imaging is be wrote, in which a resistive H-magnet is used with suitable pole pieces. In this known system there is indeed the possibility of a transverse one, but not however, free axial access to the examination volume. Because of the massive Pole shoes usually have severe eddy current problems. The homogeneous measure gnetfeld is essentially not by the geometry of the coils, but by the Shape of the pole shoes creates what the variation possibilities in the design of the Magnetic field significantly restricted. Compared to so-called "air coil arrangements", which also includes the magnet system cited above according to DE 39 07 927 A1 the pole shoe electromagnet system according to DE 36 16 078 C2 is one completely different magnet types, which, according to its peculiarities, have completely different problems having.

From EP 0 011 335 B1 a magnet coil arrangement is known, which under the Ma gnettypus "air coil" falls. It is a double Helmholtz arrangement with at least four current-carrying individual coils, in which the currents are parallel during operation flow through the coils. This known arrangement represents the geometry of the Solenoids only have an extremely narrow gap for side access which could practically not be used by a therapist.

Finally, the coil arrangement known from DE 32 45 944 C2 is concerned also a double Helmholtz arrangement with at least four current-carrying ones Individual coils in which the current flows through the coils in the same direction during operation. These be Known arrangement can no trans due to the iron shield provided there versalen access to, since the shield is closed on all sides.

In contrast, DE 39 07 927 A1 cited at the beginning describes an air coil system in Double Helmholtz arrangement, in which the four sub-coils provided with counter-rotating currents, which is a much larger spacing of the coils allows pairs in the axial direction, so that a relatively large transverse access from the page on the volume of the investigation remains free. This arrangement would make one Therapists, in particular an operator, in fact allow direct loading act of the examined patient while the treatment effect by simultaneously transferring the recorded tomograms to a monitor in Visibility of the therapist can be monitored. A disadvantage of the magnet system according to DE 39 07 927 A1, however, is that the coil arrangement is relatively com is complicated and that through the opposite loading of the coils with currents to Er generation of the magnetic field significantly higher ampere turns are required than  in the above-mentioned double Helmholtz arrangements with parallel current loading kung, as for example from EP 0 011 335 B1 or DE 32 45 944 C2 are known. Particularly in the case of resistive magnetic coils, this is also a significantly higher level here electrical power required to generate the magnetic field.

In contrast, the object of the present invention is to provide a magnet system to introduce the type mentioned, in which the coil arrangement compared to the known th arrangements should be simplified and to generate the homogeneous static Magnetic field in a comparable examination volume with the same field strength lower ampere turns, or a lower electrical power and thus ge less requirement for cooling the arrangement should be required.

According to the invention, this object is achieved in that two ferromagnetic rings (R₁₁, R₁₂; R₂₁, R₂₂) are provided coaxially to the two magnetic coils (S₁; S₂), which are arranged closer to the center of the examination volume than the magnet coil adjacent to them ( S₁; S₂), whereby for the z positions z _R11 , z _R12 , z _R21 , z _{R22 of} the rings (R₁₁, R₁₂; R₂₁, R₂₂) applies:

| z _Rÿ | <| z _Si | and | r _Rÿ | <| r _Si |

with i, j = 1, 2 and that
the rings (R₁₁, R₁₂; R₂₁, R₂₂) are positioned and dimensioned such that the magnetic field in the center of the examination volume at operating current through the magnetic coils (S₁, S₂) at least approximately up to the 10th order, preferably up to the 12th order, is homogenized.

The average distance r _{Si of} the coils S _i from the z-axis or the average distance r _{ÿ of} the rings R _ÿ from the z-axis is in each case from the center of gravity of the corresponding cross-sectional areas of the coils or rings in one of the z-axis outgoing half-plane determined. The same applies to the determination of the respective mean z positions z _Si , z _Rÿ .

The following preferably applies to the outer rings:

0.6 | z _Si | <| z _Ri1 | <| z _Si | and 0.6 | r _Si | <| r _Ri1 | <| r _Si |

or for the inner rings:

| z _Ri2 | <| z _Ri1 | and 0.6 | r _S1 | <| r _Ri2 | <| r _Si |.

Compared to the arrangement according to DE 39 07 927 A1 are in the inventive Magnet system only two current-carrying coils required, which be in the same direction be sent. This is an optimal use of the electrical power, or Amperewound number used for field generation guaranteed. The well-known Arrangement additionally required coils are in the arrangement according to the invention to a certain extent replaced by ferromagnetic rings. The ring positions below however differ significantly from the coil positions. In this way the Ma gnet arrangement significantly reduced and the relatively expensive coil material (copper and other electrically highly conductive materials) is reduced.

A particularly good axial access to the examination _volume is obtained in an embodiment in which the minimum inner diameter d _iRÿ of each ring (R _ÿ ) is in each case greater than 50%, preferably greater than 80% of its maximum outer diameter d _aRÿ .

An embodiment of the magnet system according to the invention is particularly compact, in which the magnetic coils (S₁, S₂) are wound on coil carriers and two rings (R₁₁, R₂₁) are at least partially integrated into the coil carrier. The bobbin ger completely or only partially consist of ferromagnetic material and the geometric conditions defined above for the two rings (R₁₁, R₂₁) he to fill.

An embodiment of the invention is particularly preferred in which an integrated Iron shielding is provided around the magnet system. This can on the one hand Stray field of the magnet to be shielded from the outside, but on the other hand, too the examination volume in which the magnetic field generated is particularly homogeneous must be shielded against external interference fields. In addition, iron shielding that with the same current through the solenoids, the field generated in the investigation lumen has an even higher field strength.

An advantageous development of this embodiment provides that the iron shield The central area must have at least one continuous line around the z-axis has the annular gap, so that in addition to the axial access between the ferromagneti an almost all-round transverse access is made possible. The shield mung can then be arranged in particular in the manner of two half-shells, so that the closed magnetic yoke is missing. Due to the development of power when operating the  A magnetic spacer must be provided, but it is not ferroma is genetic. Due to the continuous annular gap in the central area, a be particularly good transverse access with full rotational symmetry of the ferromagnetic Shielding enables, the rotational symmetry a particularly high field homo in the volume of the examination.

Another embodiment of the magnet system according to the invention has a fara day shielding for radio frequency fields, at least partially in the iron shield is integrated to keep the overall arrangement as compact as possible.

An embodiment is preferred in which the ferromagnetic rings (R _ÿ , i, j = 1, 2) are at least partially permanent magnets. With a smaller cross-sectional area of the rings, the same field strength can thus be generated in the examination volume as with incompletely saturated soft magnetic rings.

The rings and possibly the shield preferably have the smallest possible electrical trical conductivity, to form eddy currents when switching magnet to prevent field gradients and the resulting disturbance of the field homogeneity.

In particular, in a further development of this embodiment, the rings and if necessary, the shield is slotted, laminated or, for example, from a powder mill be pressed.

With a symmetrical and therefore particularly simple design tion form are the ferromagnetic rings of the magnet system according to the invention Circular rings. This also ensures the highest possible homogeneity of the static magnet fields in the investigation volume.

For example, for applications in mammography when both breasts at the same time to be examined, an embodiment is particularly adapted in which the fer Romagnetic rings and the main coils elliptical or approximately rectangular To have shape.

The innovation according to the invention can be particularly advantageous for resistive Ma use magnetic coils. Compared to the known systems cited above, for example the arrangement according to DE 39 07 927 A1, consumes the inventive System for generating a homogeneous magnetic field of comparable field strength significantly lower electrical power, is much lighter, more compact and contains  less copper. The magnet system according to the invention can, however, also be supralei tend solenoids included, then due to the lower required Ampere winding number when generating a comparable magnetic field strength Coils can be dimensioned significantly smaller than in known arrangements.

The strength of the homogeneous static magnetic field that can be generated is preferably un dangerous 0.1 Tesla to 0.5 Tesla (corresponds approximately to a proton frequency between 5 MHz up to 20 MHz). In this field strength range is the power saving, especially at Use of resistive magnetic coils, particularly high and dimensioning Particularly simple, since it only weakly depends on the operating field strength in this area hangs. It is even possible to operate at different field strengths, because in this Area the ferromagnetic rings are not yet magnetically saturated and therefore you proportionate contribution to the overall field essentially only from their positioning and Shape (demagnetization factor) and hardly depends on the field strength. The Remaining field dependencies can be found in any tomography system existing shim coils can be balanced.

An embodiment of the magnet system according to the invention is particularly preferred in which a magnetic field homogeneity <10 ^-4 can be achieved in an examination volume with a diameter <0.3 m, so that in a particularly large examination room, sufficiently high-resolution sectional images of the measurement object are still or the examined patient can be generated.

The z-axis of the magnet system according to the invention can preferably ver horizontally run so that the patient to be examined lies with a whole-body tomograph can, while using a transverse or axial access, the examiner Therapist or surgeon can stand or sit. In an alternative embodiment form can also run vertically, z. B. for orthopedic examinations under pressure.

The magnet system can also contain more than four ferromagnetic rings, thereby the field homogeneity can be further improved, however in general at the expense of free transverse access. It should also be noted that for relatively large z values of the rings, the field in the center is reinforced by them, but for small z values weakened. However, permanent magnetic rings could be magnetized in opposite directions so that this also makes a positive contribution in the center, even for small z-values can deliver.  

Although two solenoids are sufficient, one could use them in embodiments of the Magnetic system according to the invention by additional in the same or opposite direction complement each other, for example by more or less formal splitting of one coil each two sub-coils or by adding opposing coils with very small ones Ampere winding numbers for the marginal further improvement of the homogeneity, whereby but the main contribution to the field is still from the two main coils and the ma gnetized ferromagnetic rings comes.

By using the ferromagnetic rings, the magnet system can be preferred be built very short. It becomes quite so with a whole body system possible, lengths of less than 1.2 m, even less than 1.05 m and in extreme fall, if you accept a small ripple in the homogeneity volume, even of about 0.85 m. This represents a huge improvement over that up previous state of the art. Installation problems are mitigated, claustrophobia avoided and improved axial access.

The diameter of the system can preferably be less than 1.5 m or even less 1.3 m can be kept, which makes the device very compact and the space problems further reduced.

By effectively using the contributions of the ferromagnetic rings, the Ge total weight of preferred embodiments, even including an integrated iron bar shielding can be kept low, for example below 55,000 N or even below 32,000 N, which of course reduces transportation and installation problems.

In particular, the weight of the magnetic coils can advantageously be reduced because the ferromagnetic rings make a significant contribution to the field and the overall arrangement is more effective. The wire weight is preferably Magnetic coils less than 21,000 N, in a particularly preferred embodiment less than 13,000 N.

In the case of an extremely compact system, there is little need for homogeneity and free access even a coil weight of less than 8000 N and a Ge total weight of less than 22,000 N can be achieved.

In the case of resistive systems, this also reduces the power consumption and the cooling problems in addition to the material costs. With superconducting systems, expensive superconducting wire is saved, and the lighter and more compact coils also make cryostat construction easier. With superconducting materials, there are preferably
magnetic coils also at least some of the ferromagnetic rings within a cryostat at low temperature.

There is preferably an axial access of at least 0.5 m or at least 0.6 m, d. H. the inner diameters of the coils and rings are also larger than this value. There inserting a patient is just still possible in application forms.

In embodiments with partially interrupted or completely omitted iron shielding, there is advantageously a transverse access between the ferromagnetic rings, ie the outer rings R _i1 are at a distance of at least 0.4, preferably 0.5 m, the two inner R _i2 one of at least 0.2 m, preferably 0.24 m. In this way, a patient can be observed or treated during the examination or the access can be used for supply lines. In addition, the risk of claustrophobia is reduced because the patient can look outside (which requires reasonably transparent gradient and RF systems).

Finally, it is advantageous to compensate for assembly errors and manufacturing tolerances liable if the magnetic coils and / or the ferromagnetic rings, especially at the resistive embodiments are mechanically adjustable. You can do this in one Frame be mounted so that translational, rotary and tilt degrees of freedom exist.

Of course, includes a complete magnetic system for magnetic resonance imaging further details and additional components such as gradient systems, shim systems, Power supply units, RF devices, etc., which are known to the person skilled in the art from the general state of the art Technology are common and are therefore not discussed in detail here. May be mentioned only that gradient and RF system adapted to the task at hand and, on the one hand, preserve the access options as possible and on the other hand the spaces left between the coils and rings in the actual magnet system take advantage of as much as possible.

Further advantages of the invention emerge from the description and the attached one Drawing. Likewise, those mentioned above and those which have been elaborated further can be used Features according to the invention individually for themselves or for several in any Combinations are used. The mentioned embodiments are not as to understand the final list, but rather have exemplary character ter.

The invention is shown in the drawing and is based on concrete execution examples described and explained in more detail. Show it:

Figure 1 is a schematic, the z-axis containing longitudinal section of an inventive magnet system.

Fig. 2 is a schematic, the z-axis longitudinal section of an embodiment of a magnet system according to the invention with a transverse access possibility;

Fig. 3 is a perspective view of a yoke, which enables the transverse access;

Fig. 4 is a schematic, the z-axis longitudinal section of an embodiment of a magnet system according to the invention with axial access;

Figure 5 is a representation of the field lines in one quadrant at a half-shell-shaped ferromagnetic shield.

Fig. 6 is an illustration of a magnet with vertical direction of the z-axis;

Fig. 7 the same a computed graph of lines field distortion 1,100 ppm of a homogeneous magnetic field in 12th order in an inventive magnet arrangement.

The magnetic coil system according to the invention for nuclear magnetic resonance (NMR) tomography shown in Figure 1 has on both sides of an examination volume 1 coaxially arranged with respect to a z-axis magnet coils S₁, S₂; which are operated by direct current during operation. Coaxial to the two magnetic coils S₁, S₂, two ferromagnetic rings R₁₁, R₁₂ or R₂₁, R₂₂ are provided, which are arranged closer to the center of the examination volume 1 than the respective adjacent magnetic coil. The rings R₁₁, R₁₂ or R₂₁, R₂₂ are positioned and dimen sioned that the magnetic field in the center of the examination volume 1 is homogenized with current flow through the magnetic coils S₁, S₂ up to the 12th order.

The system shown in Fig. 1 was realized with a field strength of 0.2 Tesla, an output of the resistive, water-cooled solenoids S₁ and S₂ of 38 kW. Un investigation volume 1 , in which there is a homogeneity of the magnetic field of <10 ^-4 , has a diameter of 320 mm. The weight of the apparatus is 31,000 N, the length is only 1.00 m and the diameter is 1.28 m. The axial access has an opening of 0.55 m. Transversally, 50 cm remain free between the ferromagnetic rings R _i1 , 24 cm between the rings R _i2 . The copper coils together weigh 12,000 N.

The outer rings R _i1 are partially integrated in the coil carrier. You could also be se ready, in which case the coils lie radially completely outside the rings and can maintain their right-angled cross-section.

The system is surrounded by an integrated iron shield 2 , which in principle can also be omitted (with appropriate adaptation of the coil and ring geometries or positions). The iron shield 2 does not have to extend completely rotationally symmetrically about the magnetic axis z, but it can in general be interrupted in a symmetrical manner in the central area in order to create transverse access possibilities between the rings. In extreme cases, one or more complete ring gaps can be left free, which are only bridged by non-magnetic supports, or the shield 2 can also be completely omitted. To illustrate the size relationships, a patient 100 is drawn in on a patient bed 101 as well as a therapist 102 in FIG. 1.

A main application of the magnetic coil system according to the invention is Whole body tomography, especially mammography.

In Table 1, this magnetic system (CIMS = Compact Imaging Magnet with integrated Screen) of FIG. 1 is compared to a conventional commercial system (SIR) with four coils, which is essentially based on the concept described in DE 31 23 493 A1. With the same field strength and the same homogeneity range, the length reduced to 2/3, the total weight more than halved and the amount of copper drastically reduced to one third stand out, with approximately the same power consumption, noticeably reduced outer diameter, but also smaller inner diameter of the coils.

In Table 2, three embodiments of the invention are exemplified against each other transferred. They all have the same basic structure and the same magnetic field strength in the homogeneity volume, but differ in details because they are based on something different applications are tailored. The table is only meant to be the basic one  Illustrate the variability of the concept. However, the invention is by no means based on Limited within these examples. So, as I said, the iron shield can be removed or the field strength can be somewhat higher, especially in the case of superconducting variants lie.

The possible transverse access is illustrated in FIG. 2. Reference numerals correspond to that of FIG. 1. In the area of the transverse access, the iron shield 2 is interrupted. This can, but need not, be done over the entire scope.

Fig. 3 shows in perspective a broken magnet shield 2 in yoke shape, which allows a particularly good transverse access to the patient. The yoke 2 has a two-fold rotational symmetry with respect to the z-axis.

FIG. 4 illustrates the particularly good axial access for a therapist 102 for a particularly compact variant of the invention.

The diagram of Fig. 5 includes a quadrant of the coil ring assembly with egg nem section through the magnet coil S₁ and the rings R₁₁ and R₁₂, and a Abschir melement. 2 The course of the magnetic field lines is shown for a half-shell-shaped ferromagnetic shield 2 , which is rotationally symmetrical about the z-axis and leaves an annular gap over the entire circumference at z = 0. Such a half-shell-shaped shield is described in detail in parent application P 44 12 755.3, there in connection with FIG. 2.

In FIG. 6, an application is shown in which the magnetic axis z in the vertical direction. In this arrangement, the examination on humans under stress is possible. Examinations of the knee or the lumbar vertebrae areas under stress are particularly important for orthopedics. The magnet stands on a vertically displaceable device 103 , which on the one hand allows a comfortable entry for the patient 100 , on the other hand an exact vertical positioning of the center of the homogeneity volume. Alternatively, the patient 100 could climb in and be positioned on a vertically movable platform even with the magnet fixed.

Fig. 7 shows for a quadrant a calculated diagram of the lines of the same field deviation (especially highlighted the lines with a deviation of ± 100 ppm) in a plane perpendicular to the z-axis, the upper left corner of the image with the center of the examination volume 1 in the Apparatus collapses. The side lengths of the calculated representation each correspond to 0.35 m. In the diagram of FIG. 7 one can see six "beams", which indicates a 12th order system of field homogeneity.

In a specific case, something can deviate from a 12th order field homogeneity den, but the homogeneity is close to the exact 12th order solution should. With a so-called "overshoot" of homogeneity, for example, a Sy stem 10th order can be realized, but almost the homogeneity requirements of the 12th order system is sufficient. The advantage of such a system with "overshoot" lies in a larger radius of homogeneity, the disadvantage being a low ripple the course of the field in the homogeneity range of the apparatus can be accepted got to. Due to the less strict boundary conditions it is also possible to change the positions of the rings so far that a much better transverse access is possible becomes.  

Table 1

Table 2

Claims (26)

1. Magnetic system for nuclear magnetic resonance (NMR) tomography for generating an approximately homogeneous at least 8th order static magnetic field within an examination volume with two coils arranged with respect to a z-axis running through the center of the examination volume, arranged on both sides of the examination volume , S₂), which occupy a z-position z _S1 or z _S2 with respect to the center of the examination volume, have an average distance r _S1 or r _S2 from the z-axis and are flowed through by DC in operation in the same direction, characterized in that Coaxial to the two solenoids (S₁; S₂) two ferromagnetic rings (R₁₁, R₁₂; R₂₁, R₂₂) are provided, which are closer to the center of the examination volume than the adjacent magnet coil (S₁; S₂), for which z positions z _R11 , z _R12 , z _R21 , z _{R22 of} the rings (R₁₁, R₁₂; R₂₁, R₂₂) gi lt: | z _Rÿ | <| z _Si | and | r _Rÿ | <| r _Si with i, j = 1, 2 and that
the rings (R _ÿ ) are positioned and dimensioned such that the magnetic field in the center of the examination volume at operating current through the magnetic coils (S₁, S₂) is homogenized at least approximately up to the 10th, preferably 12th order. 2. Magnet system according to claim 1, characterized in that the following also applies: 0.6 | z _Si | <| z _Ri1 | and 0.6 r _Si | <| r _Ri1 | with i = 1, 2. 3. Magnet system according to one of the preceding claims, characterized in that the following also applies: | z _R12 | <| z _Ri1 and 0.6 | r _Si | <| r _Ri2 | <| r _Si | with i = 1, 2. 4. Magnet system according to one of the preceding claims, characterized in that the minimum inner diameter d _iRÿ of each ring (R _ÿ ) is each greater than 50%, preferably greater than 80% of its maximum outer diameter. 5. Magnet system according to one of the preceding claims, characterized in that the magnetic coils (S _i ) are wound on coil carriers and that at least two of the rings (R _ÿ ) are at least partially integrated into the coil carrier. 6. Magnet system according to one of the preceding claims, characterized in that an integrated iron shield ( 2 ) is provided around the magnet system. 7. Magnet system according to claim 6, characterized in that the Eisenschschir mung ( 2 ) in the axially central region has at least one continuous, around the z-axis annular gap. 8. Magnet system according to one of the preceding claims, characterized in that the ferromagnetic rings (R _ÿ ) are at least partially permanent magnets. 9. Magnet system according to one of the preceding claims, characterized in that the rings (R _ÿ ) have the lowest possible electrical conductivity. 10. Magnet system according to claim 9, characterized in that the rings (R _ÿ ) are slotted, laminated or pressed from powder material. 11. Magnet system according to one of the preceding claims, characterized in that the ferromagnetic rings (R _ÿ ) are circular rings. 12. Magnet system according to one of claims 1 to 10, characterized in that the ferromagnetic rings (R _ÿ ) are elliptical. 13. Magnet system according to one of the preceding claims, characterized in that the magnetic coils (S _i ) are resistive. 14. Magnet system according to one of the preceding claims, characterized net that the operating field strength of the homogeneous static magnetic field 0.1 Tesla to 0.5 Tesla (corresponds to a proton resonance frequency of approximately 5 MHz to 20 MHz) is preferably about 0.2 Tesla. 15. Magnet system according to one of the preceding claims, characterized in that a magnetic field homogeneity <10 ^-4 can be achieved in an examination volume ( 1 ) with a diameter <0.3 m. 16. Magnet system according to one of the preceding claims, characterized net that the z-axis runs horizontally. 17. Magnet system according to one of the preceding claims, characterized net that it is suitable for operation at different magnetic field strengths. 18. Magnet system according to one of the preceding claims, characterized net that it contains more than four ferromagnetic rings. 19. Magnet system according to one of the preceding claims, characterized net that its axial length is less than 1.2 m, preferably less than 1.05 m. 20. Magnet system according to one of the preceding claims, characterized net that its diameter is less than 1.5 m, preferably less than 1.3 m. 21. Magnet system according to one of the preceding claims, characterized in net that its total weight is less than 55,000 N, preferably less than 32,000 N, is. 22. Magnet system according to one of the preceding claims, characterized in that the weight of the magnetic coils (S _i ) is less than 21,000 N, preferably less than 13,000 N. 23. Magnet system according to claim 22, characterized in that the weight of the magnet coils (S _i ) is less than 8000 N and the total weight is less than 22 000 N. 24. Magnet system according to one of the preceding claims, characterized in net that there is an axial access with a diameter of at least 0.5 m. 25. Magnet system according to one of the preceding claims, characterized in that a transverse access of at least 0.4 m, preferably at least 0.5 m remains between the rings R _i1 and one of at least 0.2 m between the rings R _i2 , preferably at least 0.24 m. 26. Magnet system according to one of the preceding claims, characterized in that mechanical adjustment options are provided for the magnetic coils (S _i ) and / or the ferromagnetic rings (R _ÿ ).